Once they are this far along, the options are very limited. First it would have to be determined what exactly is going on, infection, cancer, internal laying. Most infections of this kind don't respond to antibiotics unless they are caught very early, and they almost never are because the birds hide it so well (survival instinct). For cancers there really is no treatment. If it were internal laying, then as far along as it is, she probably would not survive surgery. I would never tell someone not to seek vet care and try treatment, if that is what you want to do. But in my experience, and I've lost a fair amount to reproductive problems, they generally don't recover. I've tried treating some of them that I thought might have been caught early enough, but I have yet to have one recover. I leave them with the flock as long as they are eating and drinking well. Once they isolate themselves, go off food and water, draw attacks by the others (and they will go after a sick or weak bird), and are obviously feeling unwell, then I don't let them suffer and I euthanize. I'm very sorry, not news anyone wants to hear. 
